The reduction of carbonyl groups is one of the most significant chemical transformations in chemistry, giving access to a plethora of products from simple starting materials.[20] The development of chiral reducing agents has given access to asymmetric products[20a], including the crucially important optically pure secondary alcohols, from prochiral ketones.[21] Commonly available reducing agents have some drawbacks. Aluminohydrides and trialkylborohydrides decompose rapidly in protic solvents, such as alcohols. NaBH4 reacts only slowly with protic solvents, but dissolves poorly in nonpolar solvents. More powerful reducing agents, such as LiAlH4, are not particularly selective or tolerant of functional groups. Transition metal-based reductants share these drawbacks: many are too expensive for large-scale syntheses, and those useful for asymmetric synthesis are difficult to prepare and handle. Reducing organic carbonyls stereoselectively/stereospecifically to give chiral alcohols is a regularly sought goal, using expensive transition metal based reductants,[43] or the CBS catalyst using boron-based reagents,[49] although one intriguing case employing trialkoxysilanes and Lewis bases has appeared.[44]
Hydrosilanes are also versatile reducing agents for a variety of organic functionalities[2] including aldehydes[3] and ketones.[4] Similar to the pervasive borohydrides, the Si—H bond is polarized towards the hydrogen allowing silanes to serve as mild sources of hydride. Silanes are readily and cheaply available, as silicon is the second most abundant element in the earth's crust. Despite this significant advantage with respect both to environmental friendliness and cost as compared to borohydrides, the synthetic community has not yet developed a widely applied, operationally simple, mild, cheap, bench top method for the reduction of carbonyl groups using silanes.
The main reason for this state of affairs is that silane reactivity is difficult to tune. While alkylsilanes (such as triethylsilane) are generally easy to handle,[5] they require forceful activation in the form of a Brønsted acid,[6] Lewis acid,[7] Lewis base,[8] or transition metal[9] in the reaction mixture; the method by which these additives catalyze the reaction varies, but their presence is vital to enhance the hydridic nature of the hydrosilane. Silanes bearing more electronegative substituents (such as alkoxy or halide) or multiple hydrides are more reactive, allowing for the development of many excellent methods, yet simultaneously making them difficult—or at least inconvenient—to handle.[10] For example, Nikinov and co-workers described a useful and economical method to reduce carbonyls to alcohols using the readily available polymethylhydrosiloxane (PMHS) with catalytic hydroxide in a sealed vial within a glovebox; this method necessitates the use of a carefully sealed reaction vessel and moisture-free techniques as the active reducing agent is the volatile and highly reactive SiR4.[11] Silane reduction of aldehydes are frequently accompanied by the formation of symmetric ethers or, particularly in the case of aryl aldehydes, deoxygenated products.[16] While methods have been developed to control the product ratios in known systems, application to novel molecules requires optimization on a case-by-case basis.
Silatranes are characterized as caged structures, in which the nitrogen atom donates its lone pair of electrons to form a pentacoordinate silicon.[29] Since their discovery in the 1960s,[30] they have been extensively studied for myriad uses.[31] 1-Hydrosilatrane (1) has been less studied than other silatranes due to its anomalous physical properties and its challenging synthesis. However, it is an ideal candidate as a reducing agent due to its pentacoordinate silicon atom and its relatively high stability with respect to other silanes.[32]
In 1976, Eaborn and co-workers reported the use of 1-hydrosilatrane (1) as a reducing reagent.[12] These reactions provided poor yield and required forcing conditions. In more detail, the reductions of aldehydes and ketones carried out by Eaborn and co-workers were all carried out at a temperature of 140-180° C., for a time of 22-72 hours. The solvents were xylene, benzene and diethylene glycol diethyl ether. The reduction carried out in diethylene glycol diethyl ether at a temperature of 180° C. with an 8-fold excess of 1-hydrosilatrane, resulted in a yield of 70%; the other two reductions of an aldehyde and a ketone produced yields of 32 and 46%, respectively. Since Eaborn and co-workers disclosed this finding, the enhanced reactivity of hydrosilatrane has been discussed several times in the literature.[2, 15]

In silatranes the lone pair of the nitrogen—fixed directly opposed to the axial substituent—has been shown to donate into the σ* orbital of the axial substituent.[13] It is presumably by this mechanism that the hydrosilatrane is activated, with the intramolecular coordination of the nitrogen playing the role of a Lewis base additive. Similar types of intramolecular activation of hydrosilanes have been demonstrated.[14] Because of their structural rigidity, silatranes exhibit remarkable stability when compared to both other pentavalent silanes and other silyl orthoesters.